One technique for extracting metal from ores and other mineral material is to heap leach the material. Lixiviation is the process for effecting contact between the ore and the leaching solution. The process is normally accomplished by circulating the solution through the stationary ore mass, a technique commonly known as “percolation.” The rate of percolation will depend on a number of factors including, for example, the particle size distribution of the ore, the depth of the leach pad, the composition of the heap, scale formation within the heap, the strength of the leaching solution (e.g., conventional leach heaps use one to four pounds of sodium cyanide per ton of water for recovering gold), the quantity of leaching solution per unit area or mass and the manner in which the leaching solution is distributed over the bed of crushed ore.
An engineered pile, or heap, of particulate material is typically constructed over an engineered liner and liquid collection system. Leach solution is then applied to the upper surface of the heap and allowed to percolate through the heap where it contacts the material and dissolves one or more metal of interest or a compound thereof into the leach solution to form a pregnant solution. The pregnant leach solution is then recovered through a solution collection system and processed to separate the dissolved metal(s).
A common problem with heap leaching is the non-uniform leaching of metals from the heap. Even after extensive leaching, some portions of the heap often remain under-leached or even substantially un-leached. For example, it is often difficult to ensure that the applied leach solution will contact all portions of the heap uniformly because of permeability variations that exist within the heap. Such permeability variations may result in preferential flow of leach solution through higher permeability portions of the heap, leaving lower permeability portions under-leached or un-leached. Differences in the physical and/or chemical properties in the ore found in some portions of the heap may be less conductive to dissolution of the metal into the leach solution and may also result in less uniform leaching.
Metals remaining in under-leached and un-leached portions of a heap following heap leach operations often represent a significant loss. Some heaps are difficult to percolate because their high clay content results in leachate pooling on the surface of the heap and/or presents a barrier between to the target particle and the leachate. For these reasons, higher clay ores may be deemed uneconomical to leach or may require alternate milling extraction technologies which are energy intensive and may be substantially more expensive per unit of metal recovered.
Some ores include calcium and iron deposits distributed through the heap that incorporate valuable metal content but which are more resistant to conventional leach solutions than other ore components and tend, therefore, to suppress metal recovery percentages. Improved dissolution of these calcium and iron ore phases can result in increased liberation of metal.
Leach solutions are typically applied to heaps through a series of emitter lines which drip leach solution on an upper surface of the heap. Because these emitter lines are subject to scale formation, polymers such as polyacrylate and phosphonates are typically added to the leach solution to suppress scale formation in the emitter lines as well as within the heap itself and the solution collection system. A leaching solution, or lixiviant, used for reclaiming gold and/or silver from a bed of low-grade ore generally constitutes aqueous solutions of sodium cyanide mixed with oxygen (air) to convert the noble metal (M) to a soluble compound NaM(CN), from which M can subsequently be recovered either by precipitation with zinc dust or aluminum powder, carbon absorption, or by electrowinning. Electrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution or liquefied to form a leach solution. A current is then passed from an inert anode through the leach solution so that the metal is deposited (electroplated) onto a cathode and thereby removed from the solution with the resulting metals being referred to as electrowon. Sulfuric acid or sulfuric acid-ferric sulfate is the principle practical copper mineral lixiviant.
After the metal ore has been pre-crushed to sufficiently small size, thereby increasing its surface area and enabling the leaching solution to reach more of the metallic-mineral particles within the ore, the crushed ore is typically heaped onto an impervious pad to a depth of ten to thirty feet (3 to 10 meters). Gold and silver ores, for example, are run-of-mine or more commonly crushed to 40-200 mesh size with the metal recovery increasing as the ore particle size is reduced. Metal ore particles that are too fine, however, have a tendency to pack together and suppress percolation through the crushed ore. The underlying pad is usually sloped with the pregnant solution being recovered for subsequent processing by perforated drain pipes or channels positioned on the impervious base, typically at the lower “toe” portion of the pad.